Affine motion prediction-based image decoding method and device using affine merge candidate list in image coding system

Abstract

An image decoding method performed by means of a decoding device according to the present disclosure comprises the steps of: configuring an affine merge candidate list with respect to a current block; deriving CPMVs with respect to CPs of the current block on the basis of the affine merge candidate list; deriving prediction samples with respect to the current block on the basis of the CPMVs; and generating a reconstructed picture with respect to the current block on the basis of the derived prediction samples, wherein the maximum number of inherited affine candidates is two, a first inherited affine candidate is derived on the basis of a left block group of the current block, and a second inherited affine candidate is derived on the basis of an upper block group of the current block.

What is claimed is: 
     
       1. An image decoding method performed by a decoding apparatus, the method comprising:
 constructing a merge candidate list for deriving motion information of subblock units of a current block; 
 deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block; 
 deriving prediction samples for the current block based on the CPMVs for the CPs; and 
 generating a reconstructed picture for the current block based on the derived prediction samples, 
 wherein the merge candidate list includes (i) inherited affine candidates including a first inherited affine candidate and a second inherited affine candidate, and (ii) constructed affine candidates; 
 wherein the first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block of the current block, 
 wherein the second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block of the current block, 
 wherein the CPs include a CP 0 , a CP 1 , and a CP 2 , 
 wherein the CP 0  is a point at a top left position of the current block, the CP 1  is a point at a top right position of the current block, and the CP 2  is a point at a bottom left position of the current block, and 
 wherein each of the constructed affine candidates is derived based on a combination of a candidate motion vector for the CP 0 , a candidate motion vector for the CP 1 , and a candidate motion vector for the CP 2 . 
 
     
     
       2. An image encoding method performed by an encoding apparatus, the comprising:
 constructing a merge candidate list for deriving motion information of subblock units of a current block; 
 deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block; and 
 encoding image information including prediction information for the current block, 
 wherein the merge candidate list includes (i) inherited affine candidates including a first inherited affine candidate and a second inherited affine candidate, and (ii) constructed affine candidates; 
 wherein the first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block of the current block, 
 wherein the second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block of the current block, 
 wherein the CPs include a CP 0 , a CP 1 , and a CP 2 , 
 wherein the CP 0  is a point at a top left position of the current block, the CP 1  is a point at a top right position of the current block, and the CP 2  is a point at a bottom left position of the current block, and 
 wherein each of the constructed affine candidates is derived based on a combination of a candidate motion vector for the CP 0 , a candidate motion vector for the CP 1 , and a candidate motion vector for the CP 2 . 
 
     
     
       3. A non-transitory computer-readable storage medium storing a bitstream generated by the image encoding method of the  claim 2 . 
     
     
       4. A transmission method of data for an image, the method comprising:
 obtaining a bitstream for the image, wherein the bitstream is generated based on constructing a merge candidate list for deriving motion information of subblock units of a current block, deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block, and encoding image information including prediction information for the current block; and 
 transmitting the data comprising the bitstream, 
 wherein the merge candidate list includes (i) inherited affine candidates including a first inherited affine candidate and a second inherited affine candidate, and (ii) constructed affine candidates; 
 wherein the first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block of the current block, 
 wherein the second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block of the current block, 
 wherein the CPs include a CP 0 , a CP 1 , and a CP 2 , 
 wherein the CP 0  is a point at a top left position of the current block, the CP 1  is a point at a top right position of the current block, and the CP 2  is a point at a bottom left position of the current block, and 
 wherein each of the constructed affine candidates is derived based on a combination of a candidate motion vector for the CP 0 , a candidate motion vector for the CP 1 , and a candidate motion vector for the CP 2 .